(1) Field of the Invention
The present invention relates to a structure of burner, and more particularly to a structure of burners arranged on both sidewalls of a heating furnace having a relatively large width for heating a material conveyed from the inlet side to the outlet side of the furnace by means of a transporting means. The burner has a structure wherein a fuel gas is jetted into a heating furnace while being sandwiched between an inner air flow and an outer air flow to form a hollow flame in the furnace.
(2) Description of the Prior Art:
The inventors have already proposed in U.S. Pat. No. 4,281,984, a burner which forms a hollow flame by fuel gas. FIGS. 4 and 5 in the accompanying drawings illustrate the burner. That is, FIG. 4 is a sectional side view of the burner, and FIG. 5 is its front view viewed from the combustion furnace side.
Referring to FIGS. 4 and 5, a proper amount of air based on the amount of supplied fuel is supplied to the burner through a supply passage 1. The supply passage 1 is branched into an inner air flow passage 2 and an outer air flow passage 3. Air flow rate controlling dampers 4 and 5 are arranged in the inner and outer air flow passages 2 and 3, respectively. A baffle 7 is arranged at the end of an inner air flow supply pipe 6 formed in the center axis portion of the burner and has a relatively large area center portion, and several number of inner air flow nozzles 8, 8' . . . are arranged in the peripheral portion of the baffle 7. An annular outer air flow supply pipe 9 is formed in the peripheral portion of the burner, and has an annular baffle 10 at the end, and the annular baffle 10 has several number of outer air flow nozzles 11, 11' . . . .
Fuel gas, which has been controlled to a proper flow rate corresponding to the load of burner, is supplied from a supply passage 12, is flowed through a fuel gas supply pipe 13 arranged between the inner air flow supply pipe 6 and the outer air flow supply pipe 9 and having an annular cross-section, and then is jetted straightforwardly into the furnace through an annular fuel gas nozzle 14 arranged between an inner air flow baffle 7 and the outer air flow baffle 10 arranged at the end of the inner and outer air flow supply pipes. That is, fuel gas is jetted while being sandwiched between the inner air flow and the outer air flow, to form a hollow flame.
The burner illustrated in FIGS. 4 and 5 has the following characteristic properties.
(1) The ratio of the inner air flow rate to the outer air flow rate can be changed, whereby the length of flame can be changed.
(2) The swirl angle of inner air flow jet and that of outer air flow jet can be set to proper swirl angles, whereby a hollow flame having a desired shape can be formed.
(3) In the burning, a hollow flame is formed, and therefore generation of NO.sub.x is very small.
(4) A perfect combustion can be carried out even in a low excess air ratio.
(5) Fuel gas can be burnt while keeping the flame stable. A large number of the burners illustrated in FIGS. 4 and 5 can be arranged on both sidewalls of a heating furnace having a large width, and the furnace can be operated while keeping the furnace temperature to a desired temperature and keeping the temperature in the width direction of the furnace to a uniform temperature. Therefore, the heating time of the material to be heated can be shortened, and the thermal efficiency can be improved.
For example, in the case where the burner illustrated in FIGS. 4 and 5 is operated under a rated load, when the swirl angle of the inner air flow jet is designed to 60.degree., and the ratio of the inner air flow rate to the total air flow rate is set to 35%, the resulting flame is a short flame having a length of 1.5 m, while when the swirl angle of the inner air flow jet is designed to 60.degree. similarly to the above, and the ratio of the inner air flow rate to the total air flow rate is set to 0%, the resulting flame is a long flame having a length of 4.5 m.
However, recent operation of a heating furnace must be carried out under various conditions.
For example, a material to be heated is heated in a heating furnace sometimes at a taking out temperature of 1,200.degree. C. or sometimes at a taking out temperature of 800.degree. C. Further, a material to be heated is sometimes supplied to a heating furnace directly from a casting site under red heat, or is sometimes supplied to a heating furnace after cooled to room temperature. Furthermore, the burning air is sometimes previously heated up to 700.degree. C. or is sometimes kept to a temperature considerably lower than 700.degree. C.
The heating furnace must be often operated under a low load of about 10% based on the rated load.
In order to adapt a heating furnace of this low load operation, some of the burners arranged on both sidewalls of the heating furnace are often stopped. However, such operation system, wherein burners are operated and stopped repeatedly, is not a desirable operation from the viewpoint of safeness, and further is complicated in its burning system, is apt to cause leakage of air, and is low in the thermal efficiency.
When it is intended to carry out a low load burning of about 10% based on the rated load by means of a burner illustrated in FIGS. 4 and 5, the following drawbacks occur. That is, the flame becomes always short, the furnace temperature becomes low in the center portion of the furnace, and a uniform heating in the furnace width direction can not be carried out.